Method for forming a composite structure

ABSTRACT

Methods and systems are provided for fabricating a composite structure. In one example, the composite structure may include a honeycomb core sandwiched between face sheets. An edge of the honeycomb core may be abraded and a top face sheet may be perforated. As such, a likelihood of delamination of the composite structure during a curing step may be reduced.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 62/760,733, entitled “A METHOD FOR FORMING A COMPOSITE STRUCTURE,”filed Nov. 13, 2018. The entire contents of the above-identifiedapplication are hereby incorporated by reference for all purposes.

FIELD

The present description relates generally to methods and systems forforming a composite structure.

BACKGROUND AND SUMMARY

Flight has become a common, and in some cases, nearly day-to-day mode oftransportation since the introduction of commercial aircrafts to theU.S. during the early 1900s. Over the last century of airborne travel,aerospace technology has seen tremendous advances, especially withregards to development of new materials for aircrafts. The manufacturingof such materials has allowed for significant reductions in aircraftweight, thereby providing considerable improvements to fuel efficiencyand performance.

Bodies, or fuselages, of modern aircrafts are formed from hybridcomposite structures that have high temperature tolerance and highstrength-to-weight ratios. The composite structures may be sandwich-likearrangements of a core material with a honeycomb structure betweenarranged layers of prepregnated (prepreg) material, or face sheets,arranged on opposite faces of the honeycomb core. The honeycombstructure may be formed from a lightweight composite material such ascarbon-fiber reinforced plastic or an expanded metal and configured as anetwork of hollow cells divided and linked by cell walls. Thesurrounding face sheets, comprising layers of prepreg material mayinclude plies of resin-impregnated fiberglass cloth or prepreg graphiteand be configured to carry in-plane shear loading while provide bendingand in-plane shear stiffness to the composite structure. The sandwichstructure may be subjected to high temperatures to cure the resinincorporated in the face sheets and lend rigidity to the structure.

The honeycomb core may be attached to the prepreg material, or facesheets, by layers of adhesive film. The adhesive film securely bonds theedges of the honeycomb core to the surfaces of the face sheets and doesso over a relatively small surface area due to the structure of thehoneycomb core. For example, Smith et al. in U.S. 2009/0072086 teaches acomposite structure with a core material bonded to an interior surfaceof a face sheet with an adhesive composed of polyamide and/or rubber.The core material of the composite structure of U.S. 2009/0072086 may becut from a large block of honeycomb material, resulting in “ragged” oruneven edges that may be difficult to secure to the face sheets. Thus,the honeycomb core is treated by sanding, machining, etc., to smooth theedges. Improved adhesion between the honeycomb core and face sheets isachieved as a result.

However, the inventors herein have recognized potential issues with suchsystems. As one example, the bonding of the smooth honeycomb core edgesto the face sheets in a thick core composite structure may effectivelytrap and seal air within the cells of the honeycomb core. Upon exposureto high temperature during curing, the trapped air may expand, causingdelamination and/or disbonding between the honeycomb core and at leastone of the face sheets thus degrading a structural integrity of thecomposite structure. Frequent episodes of delamination may incuradditional costs to the manufacturing of fuselages and decreaseproduction throughput.

In one example, the issues described above may be addressed by acomposite structure comprising a honeycomb core with a treated upperedge and a non-treated bottom edge, the bottom edge on an opposite sideof the honeycomb core from the upper edge, a top face sheet coupled tothe upper edge of the honeycomb core, and a bottom face sheet coupled tothe bottom edge of the honeycomb core.

As one example, the composite structure may include the honeycomb corewith an abraded upper edge. Abrasion of the upper edge of honeycomb coremay be adjusted to yield an edge that is smooth enough to providesufficient surface area for secure adhesion of the honeycomb core to thetop face sheet yet rough enough that the upper edge does not seal offcells of the top face sheet and inhibit exchange of air between thecells. Bonding the roughened upper edge of the honeycomb core to theface sheet with a layer of adhesive maintains tiny gaps between theupper edge and a surface of the face sheet without significantlyreducing the surface area available for adhesive bonding. The face sheetmay be perforated with a predetermined uniform pattern to vent innercells of the honeycomb core during heating. A desired strength of thecomposite structure is thus provided and a likelihood of delamination isdecreased.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of an aircraft with a fuselage formed from acomposite structure.

FIG. 2 is cross-section of an embodiment of a composite structure.

FIG. 3 shows a top surface of an assembled composite test panel.

FIG. 4 shows a perspective view of an upper edge of a honeycomb corethat may be included in a composite structure.

FIG. 5 shows an example of a sanding tool used to treat an edge of ahoneycomb core of a composite structure.

FIG. 6 shows a graph plotting an amount of material removed from ahoneycomb core by machine sanding after a number of passes, error barsrepresenting maximum and minimum measurements.

FIG. 7 shows examples of treated honeycomb cores comparing differentamounts of roughening from a perspective view.

FIG. 8 shows an example of a puncturing tool that may be used toperforate a top face sheet of a composite structure.

FIG. 9 shows a plot of temperature and pressure versus time for acontrol panel formed from a composite structure during curing in anoven.

FIG. 10 shows a plot of temperature and pressure versus time duringcuring in an oven for a panel formed from a composite structure where ahoneycomb core is hand-sanded and a top face sheet is adapted withperforations.

FIG. 11 shows a plot of temperature and pressure versus time duringcuring in an oven for a panel formed from a composite structure where ahoneycomb core is machine-sanded and a top face sheet is adapted withperforations.

FIG. 12 shows a plot of temperature and pressure versus time duringcuring in an oven for a panel formed from a composite structure where ahoneycomb core is machine-sanded and a top face sheet is adapted withlow-density perforations.

FIG. 13A shows a schematic cross-section of a composite structure.

FIG. 13B shows an exploded perspective view of a composite structure.

FIG. 14 shows a microscope image of a control panel formed from acomposite structure.

FIG. 15 shows a microscope image of a processed panel formed from acomposite structure where a honeycomb core is machine-sanded and a topsheet is perforated.

FIG. 16 shows an example of a routine for forming a thick core compositestructure.

FIGS. 1-8 are shown approximately to scale.

DETAILED DESCRIPTION

The following description relates to systems and methods for forming athick core composite structure. The composite structure may be used toform the fuselage of a small aircraft. An example of such an aircraft isshown in FIG. 1. A cross-section of an embodiment of the compositestructure, depicting a sandwich-like arrangement of face sheets and ahoneycomb core, is shown in FIG. 2. A test panel of the compositestructure, fully assembled and including the face sheets, is depicted inFIG. 3. The honeycomb core, without the surrounding face sheets, isshown from a perspective view in FIG. 4. FIG. 5 shows an example of asanding tool that may be used to treat an upper edge of the honeycombcore to allow air flow between cells of the honeycomb core. A plot of anamount of material removed from the upper edge with respect to number ofpasses of a sanding tool over the honeycomb core is given in FIG. 6.Sections of the honeycomb core subjected to various degrees of sandingare compared in FIG. 7. An example of a perforation tool that may beused to treat a face sheet of the composite structure to alleviatepressure generated inside the composite structure during heating isshown in FIG. 8. FIGS. 9-12 show graphs depicting temperature andpressure plotted relative to time for each of a control panel (no coresanding and no top face sheet perforations), a panel with a hand-sandedhoneycomb core, a panel with a machine-sanded honeycomb core and a topface sheet, and a panel with a machine-sanded honeycomb core and aperforated top face sheet. The panels described in FIGS. 9-12 may beformed from the composite structure. FIGS. 13A and 13B depict acomposite structure, shown in a schematic cross-section in FIG. 13A andin an exploded perspective view in FIG. 13B to illustrate details ofinterfaces between layers of the composite structure. Microscope imagesof cross-sections of the control panel and a treated panel of thecomposite structure are compared in FIGS. 14 and 15, respectively, toshow differences in adhesion of the panels after processing. Anexemplary routine for forming the thick core composite structure isprovided in FIG. 16.

FIGS. 1-8, 13A-15 show example configurations with relative positioningof the various components. If shown directly contacting each other, ordirectly coupled, then such elements may be referred to as directlycontacting or directly coupled, respectively, at least in one example.Similarly, elements shown contiguous or adjacent to one another may becontiguous or adjacent to each other, respectively, at least in oneexample. As an example, components laying in face-sharing contact witheach other may be referred to as in face-sharing contact. As anotherexample, elements positioned apart from each other with only a spacethere-between and no other components may be referred to as such, in atleast one example. As yet another example, elements shown above/belowone another, at opposite sides to one another, or to the left/right ofone another may be referred to as such, relative to one another.Further, as shown in the figures, a topmost element or point of elementmay be referred to as a “top” of the component and a bottommost elementor point of the element may be referred to as a “bottom” of thecomponent, in at least one example. As used herein, top/bottom,upper/lower, above/below, may be relative to a vertical axis of thefigures and used to describe positioning of elements of the figuresrelative to one another. As such, elements shown above other elementsare positioned vertically above the other elements, in one example. Asyet another example, shapes of the elements depicted within the figuresmay be referred to as having those shapes (e.g., such as being circular,straight, planar, curved, rounded, chamfered, angled, or the like).Further, elements shown intersecting one another may be referred to asintersecting elements or intersecting one another, in at least oneexample. Further still, an element shown within another element or shownoutside of another element may be referred as such, in one example.

Weight reduction is a continual goal in the aerospace industry which hasled to an incorporation of composite materials in aircraft structures.Composites are hybrid materials with improved structural propertiesarising from the combination of more than one type of material.Fiberglass, carbon fiber, fiber-reinforced matrix systems are a fewexamples of composites that have become popular materials for a varietyof transport vehicles.

Modern aircrafts, such as airplanes and helicopters, may be composedlargely of composite materials. An example of an aircraft 100 is shownin FIG. 1 with an outer housing that includes a fuselage 102, wings 104,and stabilizers 106. The fuselage 102 forms a body of the aircraft 100and is an elongate chamber, capped at a front end 108 by a roundedprotruding wall, or nose, containing a cockpit and coupled to apropeller 103. The fuselage 102 may taper at a rear end 110 that iscoupled to the stabilizers 106. The aircraft 100 may be aturboprop-driven airplane, configured to operate with greater fuelefficiency compared to a turbojet-driven airplane and may be used as asmall commuter aircraft. A largest structural portion of the aircraft100 is formed by the fuselage 102, hence a weight of the fuselage 102may greatly impact a fuel consumption of the aircraft 100. By formingthe fuselage from composite materials, an overall weight of the aircraft100 as well as degradation of exterior surfaces of the aircraft 100 overtime may be reduced.

An example of a composite structure 1302 that may be used to form acomponent that includes an inner core with a low overall density, isshown in a cross-section 1300 and an exploded view 1350 in FIGS. 13A and13B. The composite structure 1302 may be applied to formation ofaircraft fuselages or other structural parts where a balance betweentensile strength and light weight is desirable. A set of reference axes201 is provided, indicating a y-axis, x-axis, and z-axis. The compositestructure 1302 may be included in a fuselage of a small turbopropairplane, such as the aircraft 100 of FIG. 1, or various other aircraftsin which a lightweight fuselage is desirable. An interface between acore of the composite structure 1302 and a top face sheet may be treatedto decrease a likelihood of delamination, as discussed further below.

The cross-section 1300 of FIG. 13A shows a sandwich structure includinga thick honeycomb core 1304 coupled to a top face sheet 1306 and abottom face sheet 1308 arranged on opposite edges of the honeycomb core1304. The top face sheet 1306 and the bottom face sheet 1308 may beformed from a variety of prepreg composite materials, including carbon,glass, aramid fibers, etc. The honeycomb core 1304 is has a much greaterthickness 1310 than a thickness 1312 of the top face sheet 1306 or athickness 1314 of the bottom face sheet 1308. In one example, thethickness 1312 of the top face sheet 1306 may be equal to the thickness1314 of the bottom face sheet 1308 but may not be equal in otherexamples. Materials from which the top and bottom face sheets 1306,1308, and the honeycomb core 1304 may be formed will be describedfurther below with reference to FIG. 2.

An internal structure of the honeycomb core 1304 is shown in theexploded view 1350 of FIG. 13B. The honeycomb core 1304 may includehollow chambers 1316, or cells 1316, that extend along the thickness1310 of the honeycomb core 1304. Each of the cells 1316 are surroundedby cell walls 1318 and each cell enclosed by cell walls 1318 may have ahexagonal shape when viewed along the y-axis. However, other geometriesof the cells 1316 and cell walls 1318, such as oval, rectangular,pentagonal, etc., have been contemplated.

An upper edge 1320 of the honeycomb core 1304 may be in face-sharingcontact with a bottom surface of a first layer of adhesive 1322. Furtherdescription of the first layer of adhesive 1322, as well as a secondlayer of adhesive 1324, will be provided with reference to FIG. 2. Anupper surface of the first layer of adhesive 1322 may be in face-sharingcontact with a bottom surface of the top face sheet 1306 and bonds theupper edge 1320 of the honeycomb core 1304 to the top face sheet 1306.Similarly, an upper surface of the second layer of adhesive 1324 may bein face-sharing contact with a lower edge 1326 of the honeycomb core1304. A lower surface of the second layer of adhesive 1324 may be inface-sharing contact with an upper surface of the bottom face sheet1308, thereby bonding the bottom face sheet 1308 to the lower edge 1326of the honeycomb core 1304.

The upper edge 1320 of the honeycomb core 1304 is shown in FIGS. 13A and13B after treatment to impart the upper edge 1320 with a desiredtexture. Specifically, the upper edge 1320 is roughened so that theupper edge 1320 is uneven, e.g., not smooth, and does not sealinglyengage with the first layer of adhesive 1322. Instead, as depicted in anexpansion 1328, the upper edge 1320 of the honeycomb core 1304 has acoarse, irregular texture. The irregular texture of the upper edge 1320includes a plurality of hills and valleys relative to the y-axis, acrossthe z-x plane. For example, portions of the upper edge 1320 may extendupwards to form hills, slopes, ridges that are asymmetric about they-axis and other portions of the upper edge 1320 may have valleys,slopes, and dips that extend downwards and are also asymmetric about they-axis. The upper edge 1320 may have fibers of material of unevenheights, with respect to the y-axis, with a jagged geometry that variesin a non-uniform manner along the z-x plane and does not show anysymmetry along any axis.

As a result of the texture of the upper edge 1320 of the honeycomb core1304, gaps are formed between the upper edge 1320 and the first layer ofadhesive 1322, as shown in FIG. 13A. The first layer of adhesive 1322may be at least semi-rigid, maintaining a flat, smooth lower surface.When coupled to the upper edge 1302, the first layer of adhesive 1322may contact the taller portions of the upper edge 1302, e.g., the hillsand ridges, and not the shorter portions of the upper edge 1302, e.g.,the valleys and dips. Air in one of the cells 1316 of the honeycomb core1304 may flow through the gaps between the first layer of adhesive 1322and the upper edge 1302 into one or more adjacent cells 1316, allowingfluidic coupling of the cells 1316 to one another. Upon heating during acuring step, the adhesive may melt and flow into the gaps, therebysealing the gaps and blocking air flow between cells 1316.

In contrast, the lower edge 1326 of the honeycomb core 1304 is untreatedand has a smooth, even geometry across the z-x plane. The smooth loweredge 1326 may couple to the second layer of adhesive 1324 so that thereare no gaps between the lower edge 1326 and the second layer of adhesive1324. Exchange of air between the cells 1316 via an interface of thelower edge 1326 and the second layer of adhesive 1324 is inhibited by asealing engagement between the lower edge 1326 and the second layer ofadhesive 1324.

Furthermore, the top face sheet 1306 may be configured with a pluralityof perforations 1330. The plurality of perforations 1330 may be holesextending through the entire thickness 1312 of the top face sheet 1306.At least one perforation of the plurality of perforations 1330 may bepositioned directly above one of the cells, fluidically coupling airwithin the cells 1316 to air external to the composite structure 1302.It will be appreciated that the plurality of perforations 1330 as wellas the gaps between the first layer of adhesive 1322 and the upper edge1320 of the honeycomb core 1304 may be depicted at a magnified scale inFIGS. 13A and 13B relative to dimensions of the composite structure 1302for illustrative purposes. Details of the treatment of the upper edge1320 of the honeycomb core 1304 and the generation of the plurality ofperforations will be elaborated in the figure descriptions below.

A cross-section 200 of an embodiment of a composite structure 250 thatmay be used to form an aircraft fuselage is shown in FIG. 2. In oneexample, the composite structure 250 may have similar stacked layers asthe composite structure 1302 of FIGS. 13A-13B but an upper edge of ahoneycomb core of the composite structure 250 is not roughened. Thecomposite structure 250, when shown in the cross-section 200 of FIG. 2,may be substantially rectangular with a central axis 203 aligned withthe x-axis. In some examples, the composite structure 250 may bemirror-symmetric about the central axis 203 and have a thick corestructure with a sandwich-like arrangement of layers. An upper layer ofthe composite structure 250 may be a top face sheet 202 that isco-planar with an x-z-plane and may be formed from stacked plies ofprepreg carbon fiber or fiberglass cloth, the plies of prepreg materialalso co-planar with the x-z plane. A bottom layer of the compositestructure 250 may be a bottom face sheet 204, also co-planar with thex-z-plane, parallel with the top face sheet 202 and also formed fromstacked layers of prepreg material. The top face sheet 202 and bottomface sheet 204 may be arranged on opposite sides of a honeycomb core206, as shown in FIG. 4, positioned in between the top and bottom facesheets 202, 204.

A top view of a panel 300 of the composite structure 250 of FIG. 2 isshown in FIG. 3. Elements of FIGS. 3 and 4 that are in common with thoseof FIG. 2 are similarly numbered. The panel 300 is square and the topface sheet 202 forms a smaller square centered on top of the bottom facesheet 204. The top face sheet 202 is coupled to the bottom face sheet204 by side walls 302, extending from edges of the top face sheet to anupper surface of the bottom face sheet 204. The side walls 302 may beangled with respect to the y-axis and may sealingly couple the top facesheet 202 to the bottom face sheet 204 so that air inside the coredcomposite structure 250 is not fluidly coupled to air outside of thecomposite structure.

The side walls 302, top face sheet 202, and bottom face sheet 204 mayall be formed from a same material. The material comprises fibers thatare woven so that surfaces of the side walls 302, top face sheet 202,and bottom face sheet 204 have a checkered appearance, e.g. composed ofmany squares. It will be appreciated that the panel 300 shown in FIG. 3is a non-limiting example and materials with other surface patterns,textures, and compositions have been envisioned.

The panel 300 of the composite structure includes the honeycomb coreenclosed by the top face sheet 202, bottom face sheet 204 and side walls302. Turning back to FIG. 2, the honeycomb core 206 may comprise aplurality of cells 208 within a structure of the honeycomb core 206 thatare separated by cell walls 210. The cell walls 210 may be formed fromcarbon-fiber reinforced plastic or some other type of composite. Athickness 212 of the honeycomb core is much greater than a thickness ofthe top face sheet 202 and the bottom face sheet 204, the thicknessesmeasured along the y-axis. In other examples, however, the thickness 212of the honeycomb core may vary with respect to the thicknesses of thetop face sheet 202 and the bottom face sheet 204. For example, thethickness 212 of the honeycomb core may be equal to the thickness of thebottom face sheet 204. It should be appreciated that the thickness ofthe honeycomb core may vary based on application.

The cells 208 and cell walls 210 may be aligned perpendicular to the topand face sheets 202, 204. The cell walls 210 may be thinner, definedalong the x-axis, than thicknesses, defined along the y-axis, of the topand bottom face sheets 202, 204 while the cells 208 may be wider,defined along the x-axis, than thicknesses of the top and bottom facesheets 202, 204.

The cell walls 210 may be parallel, of equal thicknesses and spacedevenly apart. The cell walls 210 may be continuous sheets of compositematerial arranged in a sinuous pattern, as shown in FIG. 4. Thehoneycomb core 206 is depicted with the top and bottom face sheets andshown from a perspective view 400. A first cell wall 210 a has a sinuouspattern that winds back and forth along the x-axis while extending alongthe z-axis. A second cell wall 210 b, adjacent to the first cell wall210 a, has a similar but oppositely oriented sinuous pattern.Specifically, when first cell wall 210 a curves to the left, the secondcell wall 210 b curves to the right. At portions of the first cell wall210 a that are aligned with the z-axis, the first cell wall 210 aalternates between face sharing contact with a portion of the secondcell wall 210 b that is also aligned with the z-axis, as shown in dashedcircle 402, and face sharing contact with a portion of another adjacentcell wall on the opposite side of the first cell wall 210 a from thesecond cell wall 210 b, as shown in dashed circle 404.

The cells 208 are formed from spaces enclosed by two cell walls. Forexample, a first cell 208 a is constrained by first cell wall 210 a at aleft side and by second cell wall 210 b at a right side, each cell wallforming half of the partitioning around cell first 208 a. The highlyporous structure, e.g., formed from many air-filled cells, of thehoneycomb core 206 allows a mass of the honeycomb core 206 to be lightrelative to a strength of the honeycomb core 206 in comparison withother materials such as aluminum. When securely attached to the top andbottom face sheets 202, 204 of FIG. 2, the resulting compositestructure, e.g., the composite structure 250 of FIG. 2, may provide asealed, strong, lightweight panel that may be fabricated with a desiredcurvature or geometry.

Returning to FIG. 2, the honeycomb core 206 may be positioned betweenthe top and bottom face sheets 202, 204 with upper edges 214 of the cellwalls 210, hereafter an upper edge 214 of the honeycomb core 206,coupled to a bottom-facing surface of the top face sheet 202. Loweredges 216 of the cell walls, hereafter a lower edge 216 of the honeycombcore 206, may be coupled to a top-facing surface of the bottom facesheet 204. Layers of a film adhesive 218 may be arranged between the topface sheet 202 and the upper edge 214 of the honeycomb core 206 as wellas between the bottom face sheet 204 and the lower edge 216 of thehoneycomb core 206. The film adhesive 218 may be epoxy, polyamide orbismaleimide chemistry, or some other type of compatible bondingmaterial.

The composite structure 250, when assembled as shown in FIG. 2, may bebagged and placed under vacuum for a period of time to remove air fromthe cells 208 before undergoing a thermal process to cure the filmadhesive 218 and the resins impregnated in the prepreg materials of thetop and bottom face sheets 202, 204. During the heating process,delamination between the components of the composite structure 250 mayoccur. For example, as shown by dashed circle 220, the film adhesive 218may detach from the top face sheet 202, leaving a gap therebetween.Separation between the film adhesive 218 and the upper edge 214 of thehoneycomb core 206, shown by dashed circle 222 may also occur uponexposure to heat. The delamination of the composite structure 250reduces a structural integrity of the composite structure 250 and use ofdegraded composite structures in fuselages is undesirable.

Detachment between layers of the composite structure may be due tocomplete sealing of the cells 208 of the untreated, e.g., not roughened,honeycomb core 206 during exposure to heat. Pressure within theuntreated honeycomb core 206 at ambient temperature may be atatmospheric pressure. When heated, air trapped within each of the sealedcells 208 may expand and pressure within the sealed cells 208 may riseuntil a critical pressure is attained. At the critical pressure, the topface sheet 202 to may lift up and disbond.

A likelihood of air becoming sealed and trapped within the cells 208prior to heating may be decreased by perforating the face sheets and aneffectiveness of the perforations may be enhanced by core abrasion. Inother words, the perforations may reduce the pressure inside the cells208 of the honeycomb core 206 to prevent air gaps while treating theupper edge 214 with abrasion may allow the perforations to be effectiveover a larger area. For example, the upper edge 214 of the honeycombcore 206 may be treated so that the upper edge 214 is roughened. Whenattached to the face sheet 202 by adhesive 218, small pathways createdby the uneven surface, shown in FIG. 13A between the first layer ofadhesive 1322 and the upper edge 1320 of the honeycomb core 1304, aremaintained between the upper edge 214 of the honeycomb core 206 and thelayer of adhesive 218. These pathways allow air to flow between thecells 208 upon expansion, thus equalizing pressure between the cells208.

An internal pressure of the honeycomb core 206 may be reduced byexposing the cells 208 to vacuum, the low pressure transmitted to thecells 208 through the perforations in the top face sheet 202. Whenperforated, tiny apertures may extend through the thickness of the topface sheet 202 as well as the adhesive 218, fluidically coupling thecells 208 to air surrounding the honeycomb cores 206, thereby exposingthe cells 208 to a negative pressure environment around the compositestructure generated by a vacuum source. In other words, the internalpressure of the honeycomb core 206 is reduced enough before heating thatthe cells 208 remain under negative pressure even during heating. Thus,the internal pressure of the honeycomb core 206 is maintained lowthroughout a formation of a composite structure that includes thehoneycomb core 206 and a tendency for the top face sheet 202 to lift offthe honeycomb core 206 is reduced.

Roughening of the upper edge of the honeycomb core may be achieved byvarious techniques to produce an uneven, textured surface. One exampleof a tool that may be used to treat the upper edge of the honeycomb coreis a sanding tool 500 shown in FIG. 5. The sanding tool 500 may have acircular outer shape and comprise a planar abrasive disc 502 attached toa flat face of a sanding pad 504. Surfaces of the sanding pad 504 not incontact with the abrasive disc 502 may all be curved. The sanding pad504 may be formed from a piece of a dense foam and include a metal stem506 securely coupled to a central region of the sanding pad andextending into a thickness 508 of the sanding pad 504. The stem 506 maybe used to connect the sanding pad and attached abrasive disc 502 to a3-axis CNC for machine sanding of the upper edge of the honeycomb core.

The amount of sanding applied to the honeycomb core may be adjustedbased on a desired removal of material from the honeycomb core. Forexample, a graph 600 is shown in FIG. 6 showing results ofmachine-sanding of a honeycomb core by 3-axis CNC milling. An amount ofmaterial removed from the honeycomb core due to core abrasion isdepicted relative to a number of passes conducted by the machine. A 600in/min raster speed with 0.3 inch z-direction compression is used,sanding the honeycomb core with an abrasive disc, e.g., abrasive disc502 of FIG. 5. The abrasive disc may be used to produce a coarse, unevenfinish, rather than a smooth polished one, thus a coarse grit, such as40 grit, may be employed.

Graph 600 shows that the amount of material removed decreases withnumber of passes applied to the honeycomb core until 9 passes have beencompleted. Little difference is shown between 9 passes versus 12 passes,indicating that removal by machine-sanding has reached a threshold,reducing the honeycomb core by 0.015 to 0.020 inches. Alternatively, thehoneycomb core may be sanded by hand, using 80 grit paper coupled to amanual sanding block. The amount of material removed by hand, however,may be significantly more variable and less reproducible than bymachine-sanding.

The amount of sanding applied to the upper edge of the honeycomb core,with respect to the upper edge 214 of the honeycomb core 206 of FIG. 2,may be adjusted to balance an amount of texturing of the upper edge witha sufficient surface area of the honeycomb core to bond securely to thetop face sheet of the composite structure, e.g., the top face sheet 202of the composite structure 250 of FIG. 2. A comparison of top edges offour panels of the honeycomb core, each sanded to varying degrees, isshown in a perspective view 700 of FIG. 7. The sanding may be achievedby either hand-sanding, machine-sanding, or some other technique forroughening a surface.

The four panels of honeycomb core include a first panel 702 that is notsanded. A second panel 704 is sanded by an amount that provides adesired balance between roughening of the upper edge and bonding to thetop face sheet of the composite structure. The second panel 704 may belighter in color than the first panel 702 due to removal of materialfrom an upper edge of the honeycomb core of the second panel 704. Athird panel 706 is a honeycomb core that is sanded more than the secondpanel 704. As a result of increased removal of material from an upperedge, the third panel 706 may lighter in color than both the first panel702 and the second panel 704.

The third panel 706 may be sanded to an extent where the upper edge ishighly textured and coarse. Due to the uneven texture of the upper edgeof the third panel 706, the adhesive applied between the upper edge ofthe third panel 706 and the top face sheet of the composite structuremay not bond a sufficient surface area of the upper edge of thehoneycomb core to the downward-facing surface of the top face sheet. Astrength of the composite structure may be reduced, resulting in thecomposite structure becoming less resistant to shear stresses.

A fourth panel 708 may be sanded less than either the third panel 706 orthe second panel 704. The fourth panel 708 may be darker in color thanthe second panel 704 and third panel 706 but lighter than the firstpanel 702. The reduced amount of sanding of the fourth panel relative tothe second panel 704 may allow the honeycomb core to bond securely tothe top face sheet. The relatively smooth upper edge of the fourthpanel, however, may not allow circulation and equalization of pressurebetween the cells of the honeycomb core. For example, vapors formed fromvolatiles generated during a chemical curing of a resin used to bond thecomposite structure may accumulate unevenly within individual cells ofthe honey core. Although the honeycomb may be placed under low pressureduring heating, curing of the resin may seal perforations of the topface sheet. If the cells of the honeycomb core are not maintained influidic communication with one another, variations in internal pressuresbetween the cells may allow some of the cells to have higher pressuresthan others. Confinement of vapors within individual cells may lead tonon-uniform internal pressure within the cells during heating andincreasing a likelihood that some cells may burst, forcing separatingbetween components of the composite structure.

In other words, a significant temperature gradient may be present in thehoneycomb core due in part to size and geometry. Heating during curingcreates volatiles (arising from the chemical reactions in the resin aswell as water turning into vapor). When the honeycomb core cells areunder negative pressure, the volatiles may be absorbed by the vacuumgenerated when the cells are perforated and exposed to an externalvacuum source. The internal pressure of the honeycomb core is negativethroughout the cure cycle, removing opportunities for the face sheets tolift and disbond and/or delaminate. By texturizing the honeycomb core,gaps may be formed between an upper edge of the honeycomb and the topface sheet, allowing vapors to flow between cells and reducing pressuredifferentials between cells.

When the sanded honeycomb core is assembled between the top and bottomface sheets and secured with adhesive, the top face sheet may beperforated using a tool such as a puncturing tool 800, shown in FIG. 8.The puncturing tool 800 may be an elongate, rod-like device with arotating disc 802 arranged at a first end 806 of the puncturing tool800, the rotating disc 802 configured with pins 804 extending radiallyoutwards around a perimeter of the rotating disc 802. The rotating discmay be formed from a metal or metal alloy with high tensile strength,such as steel, tungsten carbide, or titanium, and the pins 804 may be alength that allows the pins 804 to pierce through an entire thickness ofa material. For example, the pins 804 may be 4 mm long so that the pins804 may puncture through 6 plies of the material of the top face sheet(e.g., the top face sheet 202 of FIG. 2) as well as the layer ofadhesive securing the top face sheet to the upper edge of the honeycombcore. However, it will be appreciated that the puncturing tool 800 shownin FIG. 8 is a non-limiting example and other lengths of the pins 804,thicknesses of the pins 804, diameter of the rotating disc 802, andnumber of pins 804 coupled to the rotating disc 802, may be desirabledepending on a thickness of the top face sheet or geometry of thehoneycomb core. The rotating disc 802 may rotate around a bolt 808,extending through a center of the rotating disc 802 and coupling therotating disk 802 to a stem 810 of the puncturing tool 800.

The stem 810 may be a long, narrow piece of metal, such as stainlesssteel, that is curved to resemble an “S” rotated clockwise by 90degrees. The stem 810 extends between the first end 806 of thepuncturing tool 800 and a second end 812 of the puncturing tool 800 andcouples the first end 806 to the second end 812. The second end 812 ofthe puncturing tool 800 may be a handle, formed from a plastic, rubber,or a composite material and may have a curved geometry, contoured to fitcomfortably in a user's hand. Thus, the puncturing tool 800 may be ahand-held tool, gripped at the second end 812 and contacting an upwardfacing surface of the top face sheet of the composite structure (e.g.,the composite structure 250 of FIG. 2) at the first end 806 of thepuncturing tool 800. However, in other examples, the puncturing tool 800may be adapted to be an automated, motorized device.

By exerting a downward force on the puncturing tool 800, the pins 804 ofthe puncturing tool 800 may pierce through the top face sheet and upperlayer of adhesive of the composite structure. The puncturing tool 800may be rolled in contact with and along the top face sheet, forming atrail of perforations according to a direction that the puncturing tool800 is rolled. The perforations fluidically couple air inside the cellsof the honeycomb core to air outside of the composite structure,providing channels through which air inside the cells may be evacuatedby exposure to vacuum. When the air expands during a subsequent curingprocess, the increased volume of air is taken up by the vacuum (e.g.,the pressure becomes less negative). Also, the air volume inside thehoneycomb core is less when vacuum is applied, thus a mass of theexpanding air is reduced. The amount of air aspirated out of the cellsmay be adjusted by varying the number of perforations formed in the topface sheet. For example, a 12 inch by 1 inch grid pattern ofperforations on the face sheet may sufficiently release the air in thecells without affecting the structural integrity of the face sheet andcomposite structure.

The sanding of the upper edge of the honeycomb core and perforation ofthe top face sheet of the composite structure may allow pressuredifferentials between cells to be equalized and the perforations allowair to be drawn out of the cells, reducing cell pressure to belowambient pressure. Upon heating during curing, any residual air withinthe cells may expand but may still remain at a pressure below ambientpressure. A preparation of the composite structure for curing mayinclude placing a panel of the composite structure, such as the panel300 of FIG. 3, in a bag. The bag may be sealed and placed under vacuumat ambient temperature to draw air out of the composite structure andconsolidate components of the composite structure. The bottom face sheetand honeycomb core may be bagged and vacuumed over several bagging andvacuuming cycles to remove air from the cells of the honeycomb core aslayers of the composite structure, e.g., the top and bottom face sheetsand sheets of adhesive, are added. The top and bottom face sheets maynot be bonded, e.g., secured by adhesive, to the honeycomb core untilthe final heat cure is performed after the composite structure is fullylaid up.

The bottom face sheet and honeycomb core may be removed from the bag toadd the top face sheet to the upper edge of the honeycomb core, followedby bagging and vacuuming for another period of time. Perforations of thetop face sheet may be conducted under a low pressure environment withsubsequent application of active vacuum. The composite structure panelremains inside the bag under vacuum during the final curing step wherethe panel is placed in an oven and exposed to high temperatures toactivate the cross-linking of polymer chains in a resin of the top andbottom face sheets. The curing step allows the resin in the top facesheet to seal the perforations closed, thus isolating the inner volumeof the composite structure from the surrounding atmosphere.

If air is trapped within the cells of the honeycomb core, expansion ofthe trapped air during curing may generate an increase in pressurewithin the cells that leads to delamination of the composite structure.By sanding the upper edge of the honeycomb corecell-to-cell-transmission of air is allowed, equalizing pressure betweencells. Perforating the top face sheet allows air in the cells of thehoneycomb core to be removed during the bagging and vacuuming step, thusreducing a likelihood of delamination and disbonding during curing. Aneffectiveness of the core abrasion and perforation techniques are shownin a test comparing a control panel, a hand sanded panel, amachine-sanded panel with a high density spacing of perforations, and amachine-sanded panel with a low density spacing of perforations, ingraph 900 of FIG. 9, graph 1000 of FIG. 10, graph 1100 of FIG. 11, andgraph 1200 of FIG. 12, respectively.

The test may comprise drilling a hole through either the top face sheetor bottom face sheet to feed a thermocouple pressure transducer throughthe hole to measure a pressure of the air within the honeycomb core. Thehole is sealed to block air flow through the hole. Pressure in the bagis similarly monitored. The thermocouple pressure transducer is activeduring the bagging and vacuuming process as well as the curing stepwhere the composite panels are heated in an oven. Changes in pressurewith temperature in the bagged control panel that has not beenperforated or core sanded is depicted in graph 900 of FIG. 9.

Graph 900 includes an internal temperature of the control panel (plot902), a temperature inside the bag and outside the panel (plot 904), aninternal pressure of the honeycomb core of the composite structure (plot906), and a pressure inside the bag and outside of the control panel(plot 908), all relative to time, in minutes. The temperature in the bag(plot 902) is ramped quickly to 180° F., between 0-51 min, and is heldat a constant temperature until t=263 min. The temperature is ramped to210° F. at t=264 min, and ramped again to 260° F. at t=350 min. Thetemperature is then held constant until t=508 in, ramped downthereafter. The temperature of the honeycomb core (plot 904) shows asimilar profile.

The air inside the control panel is initially under vacuum, (e.g., belowzero) and increases nonlinearly as the temperature rises until t=263min. The pressure in the control panel reaches atmospheric pressure att=263 min, and continues to rise above atmospheric. The increase inpressure occurs stepwise, the steps coinciding with the temperatureramps between 263-350 min until the temperature of the control panelreaches 265° F. The pressure is at a maximum at this point and decreasesslightly while the temperature is maintained at 265° F. The pressuredecreases quickly as the control panel cools. The pressure in the bag,and surrounding the control panel, remains low, and is relativelyuniform over the duration of the test.

The rise of the pressure in the honeycomb core of the control panelabove atmospheric level may be higher than a tolerance of the compositestructure to inner pressure. For example, the adhesive may not maintainbonding between the face sheets and the honeycomb core when a pressurethreshold is surpassed. The composite structure may experiencedelamination and treatment of the composite structure according tomethods described above may be desirable. An example of results of suchtreatment is illustrated in graph 1000 of FIG. 10.

Graph 1000 shows plots of temperature and pressure versus time for acomposite panel with a honeycomb core that is hand-sanded and perforatedwith a tool such as the puncturing tool 800 of FIG. 8, followed bybagging and vacuuming the hand-sanded panel after assembly. Theperforations are formed by a tool such as the puncturing tool 800 ofFIG. 6, evenly distributing the perforations across a surface of topsheet of the composite panel and spaced four inches apart and fourinches away from the thermocouple pressure transducer. An internaltemperature of the hand-sanded panel is shown at plot 1002 and atemperature of the air inside the bag and outside of the panel is shownat plot 1004. The temperature profiles are similar to the temperatureprofiles of FIG. 9, showing analogous ramps and plateaus.

An internal pressure of the honeycomb core plot 1006 increases astemperature rises, more rapid increases coinciding with ramps intemperature. The change in pressure between 54-270 min, unlike plot 906of FIG. 9, builds more gradually with a linear relationship betweenpressure and time. Also in contrast to plot 906 of FIG. 9, plot 1006increases slightly between 344-507 min as the temperature is heldconstant at 265° F. to facilitate curing of a resin in the top andbottom face sheet. As the resin cures, the resin seals the perforationsin the top of bottom face sheet as well as gaps in a sanded edge of thehoneycomb core. The temperature reaches a maximum at 508 min before theheat is ramped down and pressure decreases. The pressure in the bagsurrounding the hand-sanded panel remains lower than inside the paneland relatively uniform.

The pressure in the hand-sanded panel does not reach atmosphericpressure (e.g., zero) even when the pressure reaches the maximum. Thevacuum initially created before the panel is heated acts as a reservoirand offsets the pressure increase. Thus, the honeycomb core remainsunder vacuum over the duration of the curing process. The results of thetest shown in FIG. 10 indicates that the process of perforating the topor bottom face sheet and sanding the honeycomb core and subjecting thecomposite structure to vacuum prior to curing is able to lower aninternal pressure of the honeycomb core enough to maintain the internalpressure of the honeycomb below atmospheric pressure throughout curing.An effect of core abrasion is further explored in test results shown inFIGS. 11 and 12.

Graphs 1100 and 1200 of FIGS. 11 and 12, respectively, also show plotsof temperature and pressure versus time. Tested composite panels ofFIGS. 11 and 12 include honeycomb cores that are machine-sanded prior toassembly of the composite structures and top face sheets of bothcomposite structures are perforated. A top face sheet of FIG. 11 isperforated with apertures spaced 4 inches apart and the panel of FIG. 11is hereafter referred to as a high-density perforated (HDP) panel. A topface sheet of the panel of FIG. 12 is perforated with apertures spaced 6inches apart and the panel of FIG. 12 is hereafter referred to as alow-density perforated (LDP) panel. The HDP and LDP panels are baggedand vacuumed before exposure to heat for curing.

Graph 1100 comprises plot 1102 showing a temperature in the HDP panel,plot 1104 showing an air temperature inside a bag containing the HDPpanel and outside the panel, plot 1106 showing an inner pressure of thehoneycomb core, and plot 1108 showing a pressure inside the bag andexternal to the HDP panel. The temperature profiles 1102 and 1104 aresimilar to each other as well as to the temperature profiles of FIGS. 9and 10.

The pressure in the honeycomb core of the HDP panel (1106) closelyresembles the pressure profile 1006 of FIG. 10, depicting similarincreases in pressure with temperature rise. The pressure reaches amaximum at t=492 min that is comparable to the maximum temperature ofplot 1006 of FIG. 10, remaining below atmospheric pressure. The similarresults of the HDP panel of FIG. 11 to the hand-sanded panel of FIG. 10indicates that machine-sanding or hand-sanding the honeycomb coreprovides comparable effects.

Graph 1200 comprises plot 1202 showing a temperature in the LDP panel,plot 1204 showing an air temperature inside a bag containing the LDPpanel and outside the panel, plot 1206 showing an inner pressure of thehoneycomb core, and plot 1208 showing a pressure inside the bag andexternal to the LDP panel. The temperature profiles 1202 and 1204 aresimilar to each other as well as the temperature profiles of FIGS. 9-11.

The pressure in the honeycomb core of the LDP panel (1206) begins undervacuum and, unlike plot 1106 of FIG. 1, increases nonlinearly between40-267 min when the temperature is held at 180° F., instead displaying acurved rise. The pressure reaches atmospheric level at t=267 min whenthe temperature is 260° F. and subsequently increases above atmosphericpressure to a pressure similar to that of the control panel in plot 906of FIG. 9.

The higher pressures generated within the honeycomb core of the LDPpanel compared to the HDP panel indicates that 6 inch spacing ofperforations in the top face sheet does not sufficiently allowaspiration of air from cells of the honeycomb core prior to heating.More densely spaced perforations, such as 4 inches spacing, may allow atleast one perforation to be positioned above each cell of the honeycombcore to reduce cell pressure enough via exposure to vacuum to reduce arise in pressure during curing resulting from expansion of residual air.

A microscope image 1400 of the control panel that has not been sanded orperforated is provided in FIG. 14 and a microscope image 1500 of amachine-sanded and perforated panel is provided in FIG. 15. The image1400 shows that adhesive layers 1402 of the control panel in themicroscope image 1400 of FIG. 14 are smooth and continuous between ahoneycomb core 1404 and a top face sheet 1406 and between the honeycombcore 1406 and a bottom face sheet 1408.

In the machine-sanded and perforated (e.g., treated) panel shown in theimage 1500 of FIG. 15, adhesive layers 1502 between a honeycomb core1504 and a top face sheet 1506 and between the honeycomb core 1504 and abottom face sheet 1508 are similar to the adhesive layers 1402 of thecontrol panel of FIG. 14. The adhesive layers 1502 of the treated panelare also continuous and smooth, indicating that the adhesive layers 1502maintain adhesion between the honeycomb core 1504 and the top and bottomface sheets 1506, 1508 despite a rougher edge of the honeycomb core 1504due to machine-sanding. Comparison of the adhesive layers of FIG. 14 andFIG. 15 suggest that sanding of the honeycomb core does not adverselyaffect cohesion of layers of a composite structure that has undergonesanding treatment.

An example of a method 1600 for fabricating a composite structure, suchas the composite structure 250 of FIG. 2 and 1302 of FIGS. 13A-13B,which may be used to form an aircraft fuselage is shown in FIG. 16. Thecomposite structure includes a lightweight honeycomb core sandwichedbetween a top face sheet and a bottom face sheet. The top and bottomface sheets may be secured to opposite edges of the honeycomb core bylayers of adhesive. The method 1600 describes a routine incorporatingtechniques to reduce a likelihood of delamination of the compositestructure during a curing process that exposes the composite structureto high temperatures. By reducing the likelihood of delamination,manufacturing costs may be decreased and production throughput may beimproved.

At 1602, the method includes forming the composite structure. Formingthe composite structure may include applying a core abrasion techniqueat 1604. Applying the core abrasion technique may comprise using anabrasive device to roughen an upper edge of the honeycomb core. In oneexample, the abrasive device may be a sanding tool, such as the sandingtool 500 of FIG. 5. The sanding tool may have an abrasive disc attachedto a foam sanding pad, configured with a connector to attach the sandingtool to a machine such as a 3-axis CNC mill. In another example, theabrasive device may be a manual sanding tool comprising coarse grit sandpaper wrapped around a sanding block.

The method proceeds to 1608 to abrade the upper edge of the honeycombcore so that the upper edge has a rough texture. For example, the upperedge may be roughened using the sanding tool adapted with a 40 gritabrasive disc. The sanding tool may be coupled to a 3-axis CNC machineand configured to pass over the upper edge of the honeycomb core twelvetimes, as shown in FIG. 6. In other examples, more or less passes overthe honeycomb core may be performed according to the coarseness of theabrasive disc and the settings of the 3-axis CNC. For example, using acoarser abrasive disc may result in conducting fewer passes to achievean equivalent roughening of the honeycomb core or adjusting the 3-axisCNC to a higher raster speed may be balanced by less passes over thehoneycomb core.

At 1610, the method continues the formation of the composite structureby applying a first layer of adhesive to a lower edge, on an oppositeside of the honeycomb core from the upper edge, of the honeycomb core.An upward-facing surface of the bottom face sheet is coupled to thelower edge of the honeycomb core at 1612 by positioning the first layerof adhesive in face-sharing contact with the first layer of adhesive andpressing the bottom face sheet against the lower edge of the honeycombcore.

At 1614, forming the composite structure further includes applying asecond layer of adhesive to the upper edge of the honeycomb core. Theabrading of the upper edge of the honeycomb core may result in formationof small gaps between the second layer of adhesive and the upper edge.The gaps may allow air to flow between cells of the honeycomb core sothat a pressure of each cell of the cells of the honeycomb care issimilar to pressures of adjacent cells. The upper edge of the honeycombcore is bonded to a downward-facing surface of the top face sheet at1616 by the second layer of adhesive, e.g., positioning the top facesheet in face-sharing contact with the second adhesive layer, andpressing the top face sheet against the upper edge of the honeycombcore.

At 1618, the method includes perforating the top face sheet of thecomposite structure. For example, a puncturing tool, such as thepuncturing tool 800 of FIG. 8, may be rolled across an upwards-facingsurface of the top face sheet while exerting downwards pressure on thesurface. Pins of the puncturing tool may puncture through the top facesheet, leaving a trail of perforations with tiny diameters. The pins maybe of a length that allows the pins to pierce through both an entirethickness of the top face sheet as well as the second layer of adhesive,thereby providing channels for air to flow between the cells of thehoneycomb core and outside of the composite structure. A spacing of thepins along a rotating disc of the puncturing tool may be adapted toyield a desired density of perforations. For example, a density ofperforations where the perforations are spaced 4 inches apart mayprovide a sufficient number of channels for air flow per cell of thehoneycomb core to effectively aspirate air out of the cells prior toheating and maintain a pressure within the honeycomb core below anatmospheric level throughout a curing process.

The method proceeds to process the composite structure at 1620.Processing the composite structure may include sealing the compositestructure in a bag, e.g., a plastic bag with high heat tolerance, at1622. The bag may be adapted with a hose coupling an interior of the bagto a vacuum pump so that air may be pumped out of the bag and thecomposite structure placed under vacuum at 1624. The bag and thecomposite structure may be maintained under low pressure for a period oftime, such as 12 hours, to allow air within the cells of the honeycombstructure to be removed via the perforations in the top face sheet ofthe composite structure. The composite structure may be exposed undermultiple cycles of vacuum to remove as much air from the cells of thehoneycomb structure as possible. As an example, each vacuum cycle maysubject the honeycomb structure to progressively lower pressure, e.g.,each successive cycle includes a stronger vacuum than a previous cycle.

Processing the composite structure may also include curing the compositestructure in an oven at 1626, after drawing air out of the compositestructure by vacuum. The composite structure, sealed in the bag undervacuum, may be heated in an oven to set resins in prepreg layers of thetop and bottom face sheets. Curing the resins may activate cross-linkingof polymer chains in the resins, increasing a tensile strength andrigidity of the top and bottom face sheets. As the resins cure duringheating, the resins may become fluid and flow into the perforations ofthe top face sheet as well as gaps between the rough upper edge of thehoneycomb structure and the downwards facing surface of the top facesheet. The perforations and gaps are sealed, blocking exchange betweeninner cell volumes of the honeycomb structure and air external to thecomposite structure as well as between the cells of the honeycombstructure. The composite structure may be exposed to temperatures up to265° F. over a period of 500 min, as shown in FIGS. 9-12. However, othercuring procedures may be used depending on a type of resin incorporatedin the top and bottom face sheets.

When the curing time has elapsed, the composite structure is allowed tocool at 1628. As the composite structure cools, the resin hardens andbecome more rigid. A tensile strength of the composite structureincreases and is maintained sealed so that the inner volume of compositestructure is not in fluidic communication with the surroundingatmosphere.

In this way, a thick core composite structure, that may be used to forman aircraft fuselage, may be fabricated to reduce a likelihood ofdelamination and disbonding during a manufacturing process of thecomposite structure. The composite structure may comprise a honeycombcore sandwiched between a top face sheet and a bottom face sheet andcoupled to the top and bottom face sheets with layers of adhesive.Treatment of the composite structure to decrease an occurrence ofdelamination may include roughening an upper edge of the honeycomb corethat is adhered to the top face sheet and may further includeperforating the top face sheet. The uneven, texture upper edge of thehoneycomb core creates gaps between the upper edge and the layer ofadhesive so that air may flow between inner cells of the honeycomb core,equalizing pressure between cells. The pressure in the honeycomb coremay be maintained low relative to atmospheric pressure by allowing airto be evacuated through perforations in the top face sheet during avacuum step to decrease an internal pressure of the composite structure.The composite structure thereby remains consolidated and intactthroughout the manufacturing process.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A composite structure comprising: a honeycomb core with a treatedupper edge and a non-treated bottom edge, the bottom edge on an oppositeside of the honeycomb core from the upper edge; a top face sheet coupledto the upper edge of the honeycomb core; a bottom face sheet coupled tothe bottom edge of the honeycomb core.
 2. The composite structure of theclaim 1, where the honeycomb core has an internal structure formed fromcells surrounded by cell walls, the internal structure extending alongan entire thickness of the honeycomb core.
 3. The composite structure ofclaim 1, wherein the treated upper edge of the honeycomb core isroughened and textured using an abrading tool to have portions formingasymmetric hills, slopes and valleys that extend upward and portionsforming asymmetric valleys, slope, and dips that extend downward.
 4. Thecomposite structure of claim 1, wherein the treated upper edge is jaggedand without symmetry along any axis.
 5. The composite structure of claim2, wherein the top face sheet is attached to the upper edge of honeycombcore by a first layer of adhesive and the bottom face sheet is attachedto the bottom edge of the honeycomb core by a second layer of adhesiveand wherein the top face sheet and the bottom face sheet are alignedco-planar with the upper edge and the bottom edge honeycomb core,respectively.
 6. The composite structure of claim 5, wherein the treatedupper edge of the honeycomb core forms gaps between the first layer ofadhesive and the upper edge and wherein the gaps fluidically couples thecells of the honeycomb core to one another.
 7. The composite structureof claim 5, wherein the top face sheet is adapted with perforationsextending through an entire thickness of the top face sheet and thefirst layer of adhesive.
 8. The composite structure of claim 7, whereinat least one of the perforations of the top face sheet is aligned with acell of the honeycomb core cells.
 9. The composite structure of claim 7,wherein air within cells of the honeycomb core are fluidically coupledto air external to the composite structure through the perforations inthe top face sheet.
 10. An aircraft comprising; a fuselage, including; acomposite structure with a honeycomb core sandwiched between a top facesheet and a bottom face sheet, the honeycomb core having a treated topedge and an untreated bottom edge, wherein the top and bottom facesheets are secured to the top edge and the bottom edge of the honeycombcore, respectively, via layers of adhesive.
 11. The aircraft of claim10, wherein the treated top edge of the honeycomb core is uneven androughened so that gaps are present between the top edge and one of thelayers of adhesive positioned between the top edge of the honeycomb coreand the top face sheet.
 12. The aircraft of claim 10, wherein thehoneycomb core includes cells, separated by cell walls, that extendthrough the honeycomb core in a direction perpendicular to a plane ofthe top and bottom face sheets.
 13. The aircraft of claim 12, whereinthe top face sheet is configured with perforations extending through anentire thickness of the top face sheet and fluidically coupled to thecells of the honeycomb core.
 14. The aircraft of claim 13, wherein thepressure in the cells of the honeycomb core is decreased by fluidicallycoupling the cells to low pressure surrounding the honeycomb core viathe perforations in the top face sheets.
 15. The aircraft of claim 10,wherein the top face sheet and bottom face sheet are formed from pliesof resin impregnated composite material and wherein the compositestructure is exposed to heat to cure the resin of the top and bottomface sheets.
 16. A method for forming a composite structure comprising;treating an upper edge of a honeycomb core; coupling the honeycomb coreto a top face sheet and a bottom face sheet to form a sandwichstructure; processing the sandwich structure to form a bonded structure.17. The method of claim 16, wherein treating the upper edge of thehoneycomb core includes abrading the upper edge to form a rough, unevenedge of the honeycomb core.
 18. The method of claim 16, wherein treatingthe upper edge of the honeycomb core includes one of abrading the upperedge with a sanding tool coupled to a 3-axis CNC and abrading the upperedge manually with sand paper.
 19. The method of claim 16, whereinprocessing the sandwich structure includes perforating the top facesheet with a puncturing tool.
 20. The method of claim 16, whereinprocessing the sandwich structure includes placing the bag under vacuumand heating the sandwich structure under vacuum in an oven to cure resinin the top and bottom face sheets.